Heavy cannabis use is associated with substantive learning and memory impairments and elevated risk of psychopathology. It has been repeatedly demonstrated that the hippocampus, centrally implicated in these processes, is particularly vulnerable to the deleterious effects of prolonged exposure to cannabis. This deterioration of hippocampal structure, function, and biochemistry can be reversed, but this requires two or more years of abstinence from cannabis. However, most heavy cannabis users find it extremely difficult to maintain abstinence over extended periods and current treatments for cannabis use disorders are inadequate. There is a pressing clinical need for an intervention that rapidly accelerates hippocampal recovery, ameliorates the associated cognitive impairments and mental health symptoms, and leads to improved treatment outcomes. One promising candidate is physical exercise. In addition to the well-known physical health benefits, regular exercise also has a potent positive effect on brain health. The current study will investitive the capacity of two different neuroscientifically-informed 12-week exercise programs can restore brain health for heavy long term cannabis users.

Heavy cannabis use is associated with substantive learning and memory impairments and elevated risk of psychopathology. It has been repeatedly demonstrated that the hippocampus, centrally implicated in these processes, is particularly vulnerable to the deleterious effects of prolonged exposure to cannabis. This deterioration of hippocampal structure, function, and biochemistry can be reversed, but this requires two or more years of abstinence from cannabis. However, most heavy cannabis users find it extremely difficult to maintain abstinence over extended periods and current treatments for cannabis use disorders are inadequate. There is a pressing clinical need for an intervention that rapidly accelerates hippocampal recovery, ameliorates the associated cognitive impairments and mental health symptoms, and leads to improved treatment outcomes. One promising candidate is physical exercise. In addition to the well-known physical health benefits, regular exercise also has a potent positive effect on brain health and can increase the size of the hippocampus. It's not yet known how much or what kind of exercise produces the best results. This study has been designed to compare the effects of two different exercise programs.

1. 12 weeks of regular HIIT exercise
2. 12 weeks of regular strength training

The research team are investigating whether the programs have a positive impact on brain health and, if they do, whether one is more effective than the other. The research team will also measure whether engaging in either program leads to a reduction in cannabis consumption, and improvements in thinking skills, mental health, and general wellbeing.

Inclusion Criteria:

1. Aged 20-55 years
2. Voluntary and able to provide informed consent
3. Fluent in English
4. Current moderate - severe cannabis use disorder
5. Major history of cannabis use (i.e. ≥3 days per week on average for ≥4 of the past 6 years)
6. Capacity to tolerate physical exercise according to 'Fitness to Exercise'

Exclusion Criteria:

1. Have a history of cardiovascular disease, high blood pressure, musculoskeletal injury or other condition that would preclude safe engagement in VO2 max fitness testing and/or regular physical exercise
2. Severe claustrophobia, non-MR compatible metallic implant, or other contraindication to MRI scanning
3. Lifetime history of significant neurological illness, or moderate - severe brain injury,
4. Current major unstable medical illness or chronic pain condition
5. Lifetime history of schizophrenia, schizoaffective disorder, OCD, PTSD, bipolar disorder
6. Current significant depression or anxiety that precludes ability to reliably engage in the exercise program
7. Current moderate - severe substance use disorder for substances other than cannabis (excluding nicotine)
8. Currently pregnant or lactating
9. Shift work employment schedule within the prior 6-months
10. Have engaged in ≥5 sessions of HITT or resistance training within the past 12-months
11. History of treatment with antipsychotic medications
12. Current participation in psychosocial treatment for substance use disorder
13. Other psychoactive medications or psychosocial treatments will be considered on a case-by-case basis. Where a current psychoactive medication is deemed acceptable, both dose and type must have been stable for a minimum of four weeks prior to baseline assessment, and remain stable throughout the 12-week exercise phase of the study.